1. Field of the Invention
This invention relates to the analysis of rocks and particularly core samples cut from subterranean sandstone formations with a nuclear magnetic resonance measuring instrument for determining the percent of the pore space that is occupied by residual water.
2. Setting of the Invention
In drilling holes in the earth for the production of oil and gas, it is common practice to obtain core samples of the subterranean formation through which the hole is drilled. These cores, as well as providing visual information as to the nature of the subterranean formation, can be analyzed to obtain important physical parameters such as porosity, permeability, fluid content, and the rock type of the formation from which the sample is cut. All rock samples taken from petroleum bearing subterranean formations contain in addition to the hydrocarbon at least a small amount of water. One important characteristic of the core sample which the petroleum engineer or geologist needs to ascertain is the percent of the pore space of the subterranean formation which is occupied by residual water. This determination of residual water can provide an indication of whether hydrocarbon production is possible in an economically viable manner. By residual water, what is meant is that water within the subterranean formation which will not flow when the pressure on the formation is reduced, such as during production through a borehole penetrating the subterranean formation. A portion of the residual water is adsorbed or otherwise bound to the internal surfaces of the subterranean formation and a portion is contained within small pore spaces. The capillary forces holding the water within the small pore spaces is too great to be overcome by the reduced pressure during production. The majority of the residual water is usually contained within the smaller pore spaces of the subterranean formation.